Acid catalysis, a common reaction mechanism in organic chemistry, implies the involvement of oxonium, or hydronium, ions, H3O+. “Acid catalysis” is inherently understood to be a process that occurs in aqueous solution. As far as the applicant is aware nobody seems to have used a gas as an acid catalyst, and with good reason. As shown in FIG. 1, gases are not ionized until very high temperatures are reached. As can be seen, there is no significant thermal ionization below 2500° C. for water and below 5000° C. for HCl. Ionization by cosmic rays and ambient radioactivity has also been shown to be negligible, together amounting to no more than 10 ion pairs/(s cm3) with a life span of 70 s. Thus, gases at all but extremely high temperatures may be considered completely nonionized, as demonstrated by their being perfect electrical insulators.
Obviously, a nonionized gas cannot be an “acid catalyst”, therefore, it has been the universal belief that acid-catalyzed processes must be carried out in the liquid phase. However, recent studies of stratospheric chemistry and the depletion of the ozone layer have shown that HCl vapour, usually stable, becomes ionized in the presence of ice crystals that are abundant in the stratosphere. HCl and water vapour molecules, are strongly adsorbed on the surface of the ice crystals. In the state of adsorption, each HCl molecules reacts preferentially with four water molecules to form an ionized cluster, H3O+ (H2O)3Cl−, in which the three water molecules form the equatorial plane of a trigonal bipyramid, with Cl− and H3O+ ions at the apexes. The chlorine atom carries a charge of −0.80 e and the oxonium ion a charge of +0.85 e, so that the electrical activity of the cluster is almost equal to that of free Cl− and H3O+ ions. The role of the solid surface is to permit HCl molecules to come into contact with four water molecules, which is not possible via collisions in a gas phase devoid of adsorbing surfaces.
It is therefore one object of this invention to provide a method for gaseous acid catalysis.
The applicant has further noted the similarity between ice crystals and other solids having multiple polar hydroxyl groups, for example sugars, and in particular pentose or pentosan.
It is therefore a further object of this invention to provide a method for gaseous acid catalysis catalysed hydrolysis of sugars to form aldehydes and in particular, pentosan and pentose to furfural.